The increasing cost of virgin pulp and the energy associated with its transformation are familiar problems to most papermakers. The boom in hardwoods utilization, the optimization of high-yield pulping processes, and the ongoing conversion to alkaline sizing are only a few examples of many attempts made in recent years to address papermaking problems. The most economically useful approach has been to replace pulp fibers with cheaper filler materials. High-filler papers are also called ultrahigh-ash paper when calcium carbonate (CaCO.sub.3) is the filler. However, the major constraint of ultrahigh-ash paper is an impairment of interfibrillar bonding. This results in decreased paper strength.
Papermaking processes often use fillers or opaque pigments to confer some desirable characteristics to the paper product and to provide a cost savings for paper raw materials. Fillers can increase opacity, brightness and printing properties. Fillers are cheaper substitutes than cellulose fibers and can reduce the total cost of the finished paper product. Moreover, fillers can be dried easier than fibers and reduce energy consumption during the papermaking process.
An essential property of paper for many end uses is its opacity. It is particularly important for printing papers, where it is desirable to have as little as possible of the print on the reverse side of a printed sheet or on a sheet below it be visible through the paper. For printing and other applications, paper must also have a certain degree of brightness, or whiteness. For many paper products, acceptable levels of optical properties can be achieved from the pulp fibers alone. However, in other products, the inherent light-reflective characteristics of the fibers are insufficient to meet consumer demands. In such cases, the papermaker adds a filler.
A filler consists of fine particles of an insoluble solid, usually of a mineral origin, suspended in a slurry. By virtue of the high ratio of surface area to weight (and sometimes high refractive index), the filler particles confer light-reflectance to the paper and thereby increase both opacity and brightness. Adding fillers to paper pulp produces an enhancement of the optical properties of the paper and further produces the advantages of improved smoothness and improved printability. Further, replacing fiber with an inexpensive filler can reduce the cost of the paper. However, filler addition poses some additional problems.
One problem associated with filler addition is that the mechanical strength of the paper is less than could be expected from the ratio of load-bearing fiber to non-load-bearing filler. The mechanical strength of paper can be expressed in terms of burst index, tear index, and tensile index. The usual explanation for this is that some of the filler particles become trapped between fibers, thereby reducing the strength of the fiber-to-fiber hydrogen bonding. The hydrogen bonding is the primary source of paper strength.
There exists a practical limit to the amount of filler which can be used. The paper mechanical properties depend primarily upon hydrogen bonding between fibrous elements. Filler accumulates on the external surface of the fibers. Accumulated filler weakens the paper strength. Further, one must use increasing amounts of retention aids to avoid excessive pigment losses through the paper-forming wire. Accordingly, filler concentrations are often limited to a maximum of about 10% ash content.
Several techniques have been used to try to overcome the problems of decreased strength from increasing filler content. Most approaches have involved filler surface modification, using retention additives, and using supplemental bonding agents. For example, preflocculated fibers and fillers have been used to increase filler retention and reduce loss of paper strength. Coarser particles of pigment or filler, caused by the preflocculation procedure, are retained more efficiently than the finer particles of pigment. Thus, there is less interference with inter-fiber bonding. This helps improve paper strength. However, paper opacity is reduced with increasing particle size. Moreover, the cost savings associated with the preflocculation technique are insignificant and are offset by additional problems.
Craig, U.S. Pat. No. 2,583,548 ("Craig"), describes a process forming a pigmented cellulosic pulp by precipitating pigment "in and around" the fibers. According to Craig, dry cellulosic fibers are added to a solution of one reactant, for example, calcium chloride, and the suspension is mechanically worked so as to effect a gelatinizing of the dry fibers. A second reactant, for example, sodium carbonate, is added so as to effect the precipitation of fine solid particles, such as calcium carbonate. The fibers are then washed to remove the soluble by-product (sodium chloride).
The Craig process has considerable limitations. The presence of filler on fiber surfaces and the gelatinizing effect on the fibers are detrimental to paper strength. The gelatinized fibers are so severely broken that both the filler precipitate and the gelled fibers form a slurry. Thus, the Craig process has not achieved commercial success despite its disclosure about 39 years ago.
Another technique is described in U.S. Pat. No. 4,510,020. This process has been called the "lumen-loading" process and it involves placing the filler material directly within the lumens of soft wood pulp fibers. "Lumen-loaded" pulp is prepared by vigorously agitating a dry softwood pulp in a concentrated suspension of filler. The action of the agitation encourages the filler to move through transverse pit apertures in the fiber cell walls and into the lumen, where the filler material is adsorbed against the surface of the lumen cavity. Subsequent washing of the lumen-filled pulp fibers rapidly eliminates residual filler from the external surfaces of the fibers but only slowly from the lumen. The result is an increased retention of filler within the lumen, while removing the hindrance to inter-fiber bonding by removing the filler outside of the fiber lumens. The result is increased paper strength for the amount of filler present. The lumen-loading technique works best with fibers that have been dried.
The lumen-loading technique, however, has not proved to be economically or commercially viable. The technique requires the manipulation of large volumes of relatively concentrated filler suspensions agitated at high revolutions for prolonged periods of time. Further, the lumen-loading technique requires a relatively small particle size filler, such as titanium oxide, which is an expensive filler material. Moreover, the lumen-loading technique will only work for dry softwood fibers having a sufficient number of pit apertures. As the lumens are open at the pits, filler may be lost in the same way that it is introduced. Further, the pores in the cell walls are not filled by the lumen-loading technique.
Accordingly, there is a need in the art to be able to produce economical paper of high opacity and strength using as much filler material as possible, and to be able to use cellulosic pulp fibers from any source (e.g., softwoods, hardwoods and annual plants, such as sugarcane).